One of the most useful scientific instruments of the 20th century, the electron microscope, continues to be used worldwide to examine the atomic world. The first transmission electron microscopes were developed in the 1930s and began seeing institutional use in the 1940s. In the 1970s, the microscope was developed further into a device that could record not just the intensity of the electrons, but their phase as well, by using coherent field-emission electron beams. Physicist Akira Tonomura, elected to the National Academy of Sciences in 2000 as a Foreign Associate, was responsible for much of this technological development.

Transmission electron microscopes beam electrons through a thin sample of material and record changes in their trajectories upon passage. The intensity of the electrons that pass through are recorded and assembled into an image. Because of wave/particle duality, the electrons have a wavelength that can be used to probe the sample, as with a light microscope. However, because the electrons have a much shorter wavelength than a photon of light, much higher resolutions are possible as compared with light microscopy.

Tonomura invented a technique to produce more coherent electron beams for greater accuracy and to precisely record electron phases, providing even more atomic information than previously possible (1). Such phase information, when added to traditional pictures of intensity, generates an electron hologram. This technique of electron holography has opened up the “pico-world,” helping to reveal and study various physical quantum phenomena. In his Inaugural Article in this issue of PNAS (2), Tonomura reviews the development of more coherent electron beams and describes the direct observation of theoretical quantum effects and the motion of magnetic vortices in superconductors, as afforded by these more powerful microscopes.

Electron Holography to the Fore

Tonomura graduated from the University of Tokyo in 1965 with a bachelor's degree in physics and has spent most of his career in Japan, working at Hitachi Laboratories. Joining Hitachi soon after graduating and working first in the Kokubunji area of Tokyo and then later in the Hatoyama area of Saitama, Japan, Tonomura has studied electronbeam physics and electron microscopy for four decades.

In 1968, Tonomura and his colleagues created the world's first electron hologram (3), for which Tonomura later won the Setoh Prize from the Japanese Society of Electron Microscopy in 1980. The resolution for this technology was not as high as current electron microscopes, and he temporarily set aside that path to concentrate on scanning electron microscopes equipped with field-emission electron guns, which provide a source for electrons. During a fellowship at the University of Tübingen (Tübingen, Germany) in 1973 and 1974, Tonomura worked with Gottfried Möllenstedt, who was the first researcher to observe electron interference patterns by developing electron biprism interferometers.

Tonomura returned to Hitachi in 1974 and turned to developing brighter electron sources through field-emission guns in order to increase the practicality of electron holography. This research led to his team's development of a highcoherence electron microscope (4). The images Tonomura and his group were producing by 1978, which recorded intensity and phase information, were detailed enough to challenge conventional electron microscopes. The key to the resolution was the high coherence of the electron beam, which was two orders of magnitude above conventional sources. When the electron wave passes through a sample, the phase is modulated by the sample's electrostatic field or magnetic field. If there is a magnetic or electrostatic field inside the material, the phase is shifted and can be used to detect and visualize the magnetic field lines of force and the electric lines of equipotentials.

“The improved electron microscope could also use the phase information to quantitatively record minute magnetic fields inside a sample that are otherwise undetectable,” he says. Traditional techniques use the information about the intensity distribution of electrons, whereas in Tonomura's method, the phase information of electrons is used in addition to the intensity distribution, which has independent information. “Therefore, using this technique, we can observe the material state in addition to material structures down to atomic dimensions,” he says. “This technique is useful for magnetism. We can directly observe microscopic magnetic lines of force.” Tonomura explains that his method can be used to observe the behavior of vortices in superconductors, or the particle and wave natures of electrons when looking at the fundamentals of quantum mechanics.

Seeking Aharonov–Bohm

Tonomura's enhanced electron microscope technique came to the forefront when it was used to search for the Aharonov–Bohm (AB) effect. In 1959, Yakir Aharonov and David Bohm proposed that a moving electron can have its phase altered by the vector potential of the electromagnetic field of a nearby object, without actually encountering the object or its magnetic field (5). James Clerk Maxwell used electromagnetic potentials as physical quantities in his union of electricity and magnetism, but the potentials came to be regarded merely as mathematical auxiliaries.

In 1981, when Tonomura was planning to make decisive experiments concerning the existence of the AB effect, he wrote a letter to Chen Ning Yang at the State University of New York, Stony Brook, a complete stranger to Tonomura at that time. He asked Yang whether this kind of experiment in fundamental physics was worth doing. “To my surprise, just one month after that, he was visiting Tokyo University and kindly visited our laboratory and discussed our planning for the experiments,” says Tonomura. “We were very encouraged by his passion and enthusiasm for the experiment. I admired his passion for physics, warm and good personality, and his ways of explaining physics in easy terms for laymen like me,” he says. Their friendship has continued to this day.

The experiment that Tonomura and his colleagues designed to detect the AB effect used a toroidal ferromagnet with a completed magnetic circuit. If the AB effect existed, two parallel streams of electrons should generate a phase difference that shows up in their interference pattern as one stream passes outside of the ferromagnet and one stream passes through its central hole. This difference could be recorded by electron holography. Tonomura and his groups indeed found that the interference pattern shifted, demonstrating the existence of the AB effect (6). The existence of the AB effect provided strong support for the popular gauge theories of the day.

Not all physicists were convinced by this experiment, however. So Tonomura's team designed another test, again with Yang's advice. This time, they fabricated a 6-μm-wide toroidal ferromagnet coated with niobium. At 5 K, niobium is a superconductor, and because of the Meissner effect, the magnetic field is completely contained within the toroid, but any vector potential would still exist outside the toroid. Again, in 1986, Tonomura was able to show a displacement of the interference fringes, conclusively demonstrating that the AB effect was real (7).

“The AB effect is very subtle,” he says, “and there are still many interpretations and implications.” Physicists may differ in their interpretation of the AB effect, but no one doubts its existence. For his observations of the AB effect, Tonomura was awarded the Nishina Memorial Prize in 1982, the Asahi Prize in 1987, and the Japan Academy Prize and Imperial Prize in 1991.

In addition to the AB effect, Tonomura and his group developed a modification of the famous double-slit experiment displaying the wave-particle duality of electrons. Instead of two slits for the electron waves to pass through, Tonomura designed a technique in which the electrons pass between an electron biprism composed of two parallel plates, with a thin filament between them.

He and his team found that when electrons were emitted once in a long while so that only one passes over the biprism at any time, it took about an hour to build up the interference pattern demonstrating the electrons' wave nature (8). In quantum mechanical terms, a single electron travels as a wave on both sides of the filament, and the interference between the two waves creates the detected pattern, which shows up as spots accumulating as particles hitting the detector one by one at seemingly random positions at first.

Swirling Vortices of Superconductors

In the 1990s, Tonomura began to study magnetic vortices in superconductors because of their physical properties, the understanding of which is needed to develop practical superconducting applications. He and his group first looked at very cold metal superconductors and later at high-temperature superconductors. Superconductors develop swirling magnetic vortices, which can be temporarily “pinned” or trapped at defects and impurities. The vortices are affected by currents, externally applied magnetic fields, and temperature.

Using electron holography, Tonomura observed the magnetic field lines emanating outside the superconductor surface from vortices inside lead superconductors in 1989 (9). In 1992, he used Lorentz microscopy, where phase shifts due to vortices are transformed into intensity variations by image defocusing, to dynamically observe vortices inside thin films of superconducting niobium (10). The key to the latter observation was his team's development of a 350-kV electron holography microscope, the bright beam of which had the required strength and resolution to make the observations. For his development of the 350-kV electron microscope and observations of the AB effect and magnetic vortices, Tonomura received the Benjamin Franklin Medal in Physics in 1999. “In order to observe vortices in high-temperature superconductors, the sample must be thicker, since the vortices are 10 times thicker than those in metal superconductors,” he says. Only a highenergy, yet bright, beam can penetrate such thickness.

“The vortices move around like living creatures.”

One Million Volts

To peer deeper into the behavior of superconductors, Tonomura continued to develop the electron holography microscope, and, in 2000, he succeeded in the creation of a 1-million-volt (MeV) microscope (11). This microscope has the brightest beam and resolution of any transmission electron microscope, with a beam brightness of 2 × 1010 Acm–2str–1 and lattice resolution of 49.5 pm, both world records. A conventional 1-MeV electron microscope with a thermal electron beam has a brightness of 108 Acm–2str–1 and 100-pm lattice resolution.

In his PNAS Inaugural Article (2), Tonomura describes the use of his 1-MeV electron microscope to record detailed real-time observations of vortices inside superconductors, revealing unusual behavior. “The vortices move around like living creatures,” he says. His team has seen the vortices drift through a superconductor with a lattice of point defects. The vortices can become trapped at these defects and then, when these locations are filled, fill the spaces in between. His team has also observed many more vortices with opposite polarizations than expected by theory.

Because vortices and antivortices collide and merge, releasing small bursts of thermal energy, Tonomura suggests that this phenomenon may be useful for studying matter–antimatter collisions because the pair-annihilation process of vortices and antivortices can be observed in real time. “They are elementary particles in the sense they cannot be divided into two,” he says. These mergers are undetectable by macroscopic magnetic field measurements, because the opposite polarizations cancel each other and the sample region where the vortex–antivortex collisions occur appears to have no magnetic field.

Antivortices have proven to play a crucial role in vortex pinning in some cases. Even when vortices are strongly trapped at local pinning centers, antivortices later formed in regions without pinning centers are attracted to and annihilate the trapped vortices. Tonomura explains in his Inaugural Article (2) that this finding shows that even strongly trapped vortices can be depinned from the pinning centers. This annihilation may represent an analog of the matter–antimatter collisions performed in the largest particle accelerators by this process. Tonomura explains that more easily observed vortex collisions may help untangle the dynamics of collisions and perhaps cosmic strings in the early universe, as well as advance the understanding of defects in other exotic materials, such as those that exist in superfluid helium.

Purely Technical Challenges

Developing brighter electron microscopes has been a “purely technical” challenge, in Tonomura's words. With the resolution of the 1-MeV microscope down to ≈50 pm, he says the only problems with getting the resolution higher are technical ones. The wavelength of 1-MeV electrons is 0.9 pm, and the main obstacles to higher resolution are having an aberration-free electron lens system, a stable platform for the microscope, a bright and monochromatic electron gun, and sensitive electron detectors.

“I hope this can be realized down to the fundamental limit,” he says. “If we obtain a resolution far below 1 Å, say 0.1 Å, or 102 pm, we can observe any light element, such as hydrogen atoms and other small molecules, even in three dimensions.” He believes a beam of 1–2 MeV will be enough. As he notes in his Inaugural Article (2), each time brighter electron beams have been developed, quantum effects deemed “thought experiments” have become more visible. Some day, many other quantum physics phenomena may be tested and revealed, if Tonomura has his way.

Footnotes

This is a Profile of an elected member of the National Academy of Sciences to accompany the member's Inaugural Article on page 14952.

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